Semiconductor laser element and method of fabrication thereof

ABSTRACT

A semiconductor laser element having an advantageous vertical light confinement efficiency, a low threshold current and a low element resistance is provided. The semiconductor laser element has a substrate and a stacked structure formed thereon, where the stacked structure comprises a buffer layer, an n-Al 0.6 Ga 0.4 As cladding layer, an n-Al 0.47 Ga 0.53 As cladding layer, an active layer, a p-Al 0.47 Ga 0.53 As first cladding layer, an Al 0.55 Ga 0.45 As etching stop layer, a p-Al 0.47 Ga 0.53 As second cladding layer, a p-Al 0.6 Ga 0.4 As third cladding layer, and a p-GaAs contact layer. The second and third cladding layers, and the contact layer are formed as a stripe-patterned ridge, and serve as a current injection regions. Both lateral portions of the ridge are filled with an n-type current blocking layer and serve as non-current-injection regions. Because the cladding layers on the active-layer-section side have a refractive index larger than that of the cladding layers disposed outward thereof, light leaked from the active layer section can efficiently be confined within the cladding layers on the active-layer-section side.

RELATED APPLICATION DATA

This application is a divisional of U.S. patent application Ser. No.10/762,696, filed Jan. 22, 2004, now U.S. Pat. No. 7,133,430 theentirety of which is incorporated herein by reference to the extentpermitted by law. The present invention claims priority to Japanesepatent application No. 2003-014451 filed in the Japanese Patent Officeon Jan. 23, 2003, the entirety of which also is incorporated byreference herein to the extent permitted by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser element and amethod of fabricating the same, and more particularly to a AlGaAs-basedridge-stripe semiconductor laser element withlow operational voltage andlow element resistance, and a method of fabricating the element.

2. Description of the Related Art

In recent years, AlGaAs-based infrared wavelength semiconductor laserelements are used as light sources for reading devices, rewritingdevices and initializers for optical disks. In particular, infraredhigh-output semiconductor laser elements of a broad-stripe type capableof oscillating lateral multi-mode laser light are expected as lightsources for exciting solid lasers such as Nd:YAG and Nd:YVO₄ havingabsorption band of the crystals at around 808 nm. Also processing toolssuch as welders based on use of the infrared high-output semiconductorlasers are becoming increasingly popular.

In these fields, the high-output semiconductor laser elements haveparticularly been on demand for realizing a still higher lightconfinement efficiency and a still lower threshold current.

Referring to FIG. 10, a configuration of a conventional AlGaAs-basesemiconductor laser element having a buried-ridge-type structure will bedescribed. FIG. 10 is a cross-sectional view of a conventionalAlGaAs-base semiconductor laser element having the buried-ridge-typestructure.

As shown in FIG. 10, a conventional buried-ridge-type AlGaAs-basesemiconductor laser element 200 includes a double-hetero-stackedstructure formed on an n-GaAs substrate 201, where the stacked structurecomprises an n-GaAs buffer layer 202, an n-Al_(0.47)Ga_(0.53)As claddinglayer 203, an active layer section 204, a p-Al_(0.47)Ga_(0.53)As firstcladding layer 205, an etching stop layer 206, a p-Al_(0.47)Ga_(0.53)Assecond cladding layer 207 and a p-GaAs contact layer 208, all of whichare epitaxially grown sequentially in this order.

The p-second cladding layer 207 and p-contact layer 208 are formed as aridge so as to constitute a current injection region 220. Both lateralsections of the ridge composing the current injection region 220 arefilled with an n-GaAs current blocking layer 211 to thereby formnon-current-injection regions 221.

A p-side electrode 212 is formed on the upper surfaces of the p-contactlayer 208 and the n-GaAs current blocking layer 211, and an n-sideelectrode 213 is formed on the back surface of the n-GaAs substrate 201.

In the semiconductor laser element 200, filling of both lateral sectionsof the ridge-stripe-patterned current injection region 220 with asemiconductor material of a conduction type opposite to the currentinjection region 220 is successful in realizing narrowing of both thecurrent and refractive-index-based waveguide at the same time.

It can thus be said that the aforementioned semiconductor laser element200 has a configuration capable of effectively confining both injectedcarriers and laser light.

The following paragraphs will describe a method of fabricating theconventional semiconductor laser element 200 referring to FIG. 11A toFIG. 13F. FIGS. 11A and 11B, FIGS. 12C and 12D, and FIGS. 13E and 13Fare cross-sectional views showing layer structures in the individualprocess steps in the fabrication of the conventional semiconductor laserelement 200.

First, as shown in FIG. 11A, the n-GaAs buffer layer 202, then-Al_(0.47)Ga_(0.53)As cladding layer 203, the active layer section 204,the p-Al_(0.47)Ga_(0.53)As first cladding layer 205, the etching stoplayer 206, the p-Al_(0.47)Ga_(0.53)As second cladding layer 207, and thep-GaAs contact layer 208 are epitaxially grown sequentially in thisorder on the n-GaAs substrate 201 in the first epitaxial growth step byan organometallic vapor phase growth process such as the MOVPE processand MOCVD process, to thereby form a stacked structure 210 having adouble hetero-structure.

In the epitaxial growth, Si, Se and so forth are used as the n-typedopant, and Zn, Mg, Be and so forth as the p-type dopant.

Next, as shown in FIG. 11B, an SiO₂ film 214 is formed on the topsurface of the stacked structure 210, that is, the upper surface of thep-GaAs contact layer 208, by a CVD (Chemical Vapor Deposition) processor the like, and further on the SiO₂ film 214, a stripe-patterned resistmask 215 is formed by photolithography.

Next, the SiO₂ film 214 is mask-patterned with the resist mask 215, andthe resist mask 215 is then removed, to thereby form an SiO₂ mask 214 onthe p-GaAs contact layer 208, as shown in FIG. 12C.

Next, the p-GaAs contact layer 208 and the p-Al_(0.47)Ga_(0.53)As secondcladding layer 207 are etched by wet etching technique under maskingwith the SiO₂ mask 214, to thereby form a ridge.

The etching is carried out using an etchant which is capable ofcompletely removing the p-GaAs contact layer 208 and thep-Al_(0.47)Ga_(0.53)As second cladding layer 207, and having an etchingselectivity enough to terminate the etching on the surface of theetching stop layer 206. This makes it possible to selectively remove thep-Al_(0.47)Ga_(0.53)As second cladding layer 207 without affecting theetching stop layer 206.

Next, as shown in FIG. 13E, the process advances to a second epitaxialstep, where the n-GaAs current blocking layer 211 is grown on bothlateral potions of the ridge. Because the SiO₂ mask 214 resides on theridge, the GaAs current blocking layer 211 does not grow on the ridge.

Next as shown in FIG. 13F, the SiO₂ mask 214 is removed, the p-sideelectrode 212 is formed on the p-contact layer 208 and the n-GaAscurrent blocking layer 211, and the n-side electrode 213 is formed onthe back surface of the n-GaAs substrate 201.

The aforementioned conventional semiconductor laser element 200 isobtained after all of the above-mentioned process steps.

SUMMARY OF THE INVENTION

The above-described semiconductor laser element 200, however, presentsthe following disadvantages as described below:

a first disadvantage is that the semiconductor laser element has a highoperational voltage and high element resistance due to structuralreasons; and

a second drawback is that the fabrication processes involved arecomplicated and are costly again due to structural reasons.

A typical problem arises in association with the ridge formation.Assumption is made now that, in the process shown in FIG. 12D, theetching stop layer 206 is composed of Al_(m)Ga_(1−m)As, and ahydrofluoric-acid-containing etchant is used in the etching for formingthe stripe-patterned ridge.

Assuming now that the etching stop layer 206 composed ofAl_(m)Ga_(1−m)As has no etching selectivity against thehydrofluoric-acid-containing etchant, the etching may proceed so as topenetrate the etching stop layer 206 to reach the p-Al_(0.47)Ga_(0.53)Asfirst cladding layer 205, and may even reach the active layer section204 depending on occasions.

Because the etchrate attainable by the hydrofluoric-acid-containingetchant depends on the Al compositional ratio, it is found necessary toset the Al compositional ratio “m” so as to allow the etching to proceedthrough the p-Al_(0.47)Ga_(0.53)As second cladding layer 207, but toterminate on the etching stop layer 206, so far as thehydrofluoric-acid-containing etchant is adopted.

Lowering the Al compositional ratio “m” so as to expand the etchingselectivity between the p-Al_(0.47)Ga_(0.53)As second cladding layer 207and the etching stop layer 206, for example, undesirably raises aproblem of increase in the element resistivity due to increase in thecarrier recombination within the etching stop layer 206.

In place of the adjustment of the Al compositional ratio “m” of theetching stop layer 206, another possible strategy is such as raising theconcentration of the hydrofluoric-acid-containing etchant aiming atincreasing the etching selectivity between the p-Al_(0.47)Ga_(0.53)Assecond cladding layer 207 and the etching stop layer 206. This, however,raises another problem of lowering in the etchrate of thep-Al_(0.47)Ga_(0.53)As second cladding layer 207, which inhibits theetching.

Therefore, one cannot help saying that it is difficult to control theetching selectivity based on concentration of thehydrofluoric-acid-containing etchant.

Another problem of the hydrofluoric-acid-containing etchant resides inthat it can etch also the SiO₂ mask 214. It is therefore necessary todetermine the concentration of the hydrofluoric-acid-containing etchantso as to ensure the etching selectivity against the SiO₂ mask 214, andit is still also necessary to adjust the thickness of the SiO₂ mask 214.In short, use of the hydrofluoric-acid-containing etchant islabor-consuming.

There is proposed an alternative method in which GaInP is used forcomposing the etching stop layer 206, and a sulfuric-acid-containingetchant is adopted as the etchant in place of thehydrofluoric-acid-containing etchant, which may successfully increasethe etching selectivity ratio, and thereby terminate the etching on thesurface of the etching stop layer 206.

Adoption of GaInP for the etching stop layer 206, however, inevitablyrequires replacement of the previous As-containing furnace atmospherewith a P-containing furnace atmosphere, and lowering of the growthtemperature in the furnace, in order to grow the GaInP etching stoplayer in the first epitaxial growth step. After the GaInP etching stoplayer was grown, the growth temperature in the furnace must be elevatedagain, the As-containing atmosphere must be recovered in order to growthe residual p-Al_(0.47)Ga_(0.53)As second cladding layer 207, and thep-GaAs contact layer 208.

This makes the crystal growth process more complex, extends theoperation time in the epitaxial growth process, and raises the costs.

It is also known that a high-output, broad-stripe-type semiconductorlaser element using the GaInP etching stop layer may largely vary thegeometry of the NFP (Near Field Pattern) as being affected by latticedistortion induced by the GaInP etching stop layer.

In view of such problems, Japanese Laid-Open Patent Publication No.5-259574 proposes a method of forming the ridge by selectively etchingthe cladding layer using an etchant comprising an organic acid andhydrogen peroxide.

In other words, the patent publication describes that use of AlGaAshaving an Al compositional ratio of 0.38 to 0.6 for the cladding layer,use of an AlGaAs layer having an Al compositional ratio of 0.6 or largerfor the etching stop layer, and use of a specified etchant is successfulin forming the ridge with a good reproducibility, and consequently inreadily fabricating the semiconductor laser element.

The above-described patent publication discloses an example in which thecladding layer is composed of Al_(0.5)Ga_(0.5)As, a 0.06-μm-thicketching stop layer is composed of Al_(0.6)Ga_(0.4)As, and the specificetchant comprises a mixed solution of tartaric acid and an aqueoushydrogen peroxide solution.

In addition, this specific etchant can etch the Al_(0.5)Ga_(0.5)As layerbut cannot etch the Al_(0.6)Ga_(0.4)As layer, so that the etchingterminates upon exposure of the Al_(0.6)Ga_(0.4)As layer, and thisensures the ridge formation with good reproducibility.

The patent publication also describes that use of the same AlGaAs layerboth for the etching stop layer and the cladding layer makes it possibleto grow the etching stop layer and the cladding layer under same growthconditions, so that only a control of the Al compositional ratio issuccessful in readily forming the cladding layer and the etching stoplayer in a succeeding manner with an advantageous crystallinity.

It is also described that the light confinement efficiency can be raisedbecause the etching stop layer is adjusted to have an Al compositionalratio larger than that of the cladding layer, and this makes it possibleto provide a region having a refractive index smaller than that of thecladding layer.

The configuration of the semiconductor laser element disclosed in thepatent publication, however, has the Al_(0.6)Ga_(0.4)As etching stoplayer having a refractive index smaller than that of theAl_(0.5)Ga_(0.5)As cladding layer on the ridge side, and this results ina problem that the light generated in the active layer is undesirablypushed out towards the opposite side of the ridge, and consequentlymakes it difficult to raise the light confinement efficiency.

Another problem resides in that increase in the thickness of theAl_(0.6)Ga_(0.4)As etching stop layer may be successful in improving theoptical characteristics, but the Al_(0.6)Ga_(0.4)As etching stop layerhaving a band gap energy larger than that of Al_(0.5)Ga_(0.5)Ascomposing the cladding layer also serves as a barrier against thecarriers, and increase in the thickness thereof may result in increasein the threshold current.

The present invention has been conceived in view of the aforementionedproblems in the prior art, and is aimed at providing an AlGaAs-basesemiconductor laser element having a large vertical light confinementefficiency, a low threshold current and a low element resistance, andalso at providing a method of fabricating the element.

A semiconductor laser element according to a preferred embodiment of thepresent invention (referred to as a first preferred embodiment,hereinafter) includes an AlGaAs-based ridge-stripe semiconductor laserelement comprising an upper AlGaAs-base cladding layer and a lowerAlGaAs-base cladding layer placing an active layer in between, whereineach of the upper AlGaAs-base cladding layer and the lower AlGaAs-basecladding layer further comprises two or more cladding layers includingan AlGaAs-base first cladding layer close to the active layer, and anAlGaAs-base second cladding layer disposed outward on the AlGaAs-basefirst cladding layer relative to the active layer and having a larger Alcompositional ratio and a smaller refractive index than the AlGaAs-basefirst cladding layer.

Because the first cladding layer of the semiconductor laser elementaccording to this first preferred embodiment of the present inventionhas a refractive index larger than that of the second cladding layer,light leaked from the active layer can efficiently be confined withinthe first cladding layer, and this raises a light confinement factor ofthe semiconductor laser element.

More specifically, the first cladding layer and the second claddinglayer are formed as an Al_(x)Ga_(1−x)As (0<x<1) layer and anAl_(y)Ga_(1−y)As (0<y<1) layer, respectively, where x<y.

A semiconductor laser element according to another preferred embodimentof the present invention (referred to as a second preferred embodiment,hereinafter) includes an AlGaAs-based ridge-stripe semiconductor laserelement having a stacked structure formed on a GaAs substrate, thestacked structure having: an Al_(y)Ga_(1−y)As (0<y<1) cladding layerhaving a same conductivity type as the substrate, an Al_(x)Ga_(1−x)As(0<x<1) cladding layer having a same conductivity type as the substrate,a non-doped active layer section, an Al_(x)Ga_(1−x)As (0<x<1) firstcladding layer having a conductivity type opposite to the substrate, anAl_(x)Ga_(1−x)As (0<z≦1) etching stop layer, an Al_(x)Ga_(1−x)As (0<x<1)second cladding layer having a conductivity type opposite to thesubstrate, an Al_(y)Ga_(1−y)As (0<y<1) third cladding layer having aconductivity type opposite to the substrate, and a GaAs contact layerhaving a conductivity type opposite to the substrate; wherein the secondcladding layer, the third cladding layer and the contact layer areformed as a stripe-patterned ridge; and an Al compositional ratio “z” ofthe etching stop layer, an Al compositional ratio “x” of the firstcladding layer and the second cladding layer, and an Al compositionalratio “y” of the third cladding layer satisfy the relations x<z and x<y,where a difference between “x” and “z” is set to 0.025 or more.

Because the Al compositional ratio “x” of the first cladding layer andthe second cladding layer and the Al compositional ratio “z” are set soas to differ only by as small as 0.025 or more, the second preferredembodiment is successful in achieving, in addition to the effect of thethe first preferred embodiment of the present invention, reduction inthe element resistance in the etching stop layer, and consequently inrealizing a semiconductor laser element having a small thresholdcurrent.

Provision of the etching stop layer as specified in the second preferredembodiment makes it possible to select an etchant having an etchingselectivity between the etching stop layer and the second cladding layerin the ridge formation, and selection of a citric acid-containingetchant makes it possible to form the ridge by a simple process with agood reproducibility.

The etching stop layer of the second preferred embodiment has the sameAlGaAs-base material with that for the cladding layer, and this makes itpossible to epitaxially grow the first cladding layer, etching stoplayer, second cladding layer, third cladding layer and contact layer ina succeeding manner.

In the etching of the second cladding layer in the second preferredembodiment, the thickness of the Al_(z)Ga_(1−z)As (0<z≦1) etching stoplayer is adjusted within a range from 0.015 μm and 0.02 μm in thepreposition that the etchant is chosen so as to ensure selectivitybetween the etching stop layer and the second cladding layer, which istypically a citric-acid-containing etchant.

The thickness of the etching stop layer is successful in reducinginfluences of the refractive index of the etching stop layer against thelight generated in the active layer, and is less influential to theoptical characteristics of the semiconductor laser element.

The thickness of the etching stop layer less than 0.015 μm results inonly an insufficient effect as the etching stop layer, whereas thethickness exceeding 0.02 μm is unnecessary in view of exhibiting theeffect of the etching stop layer, or rather results in a problem ofincrease in the threshold current.

In the formation of the ridge, it is also allowable to form a structurehaving the Al_(z)Ga_(1−z)As (0<z≦1) etching stop layer omittedtherefrom, or it is still also allowable to adopt a structure having theridge etched as deep as to reach the Al_(x)Ga_(1−x)As (0<x<1) firstcladding layer having a conductivity type opposite to the substrate.

In the second preferred embodiment, the Al compositional ratio “x” maydiffer between the Al_(x)Ga_(1−x)As (0<x<1) first cladding layer and theAl_(x)Ga_(1−x)As (0<x<1) second cladding layer.

The Al_(z)Ga_(1−z)As (0<z≦1) etching stop layer may have the same Alcompositional ratio with that of the Al_(y)Ga_(1−y)As (0<y<1) thirdcladding layer, so far as its Al compositional ratio is larger by 0.025or more than that of the Al_(x)Ga_(1−x)As (0<x<1) second cladding layer.

Another preferred embodiment of the present invention includes a methodof fabricating An AlGaAs-based ridge-stripe semiconductor laser elementcomprising the steps of forming a stacked structure on an active layersection, the stacked structure comprising an Al_(x)Ga_(1−x)As (0<x<1)first cladding layer, an Al_(z)Ga_(1−z)As (0<z≦1) etching stop layer, anAl_(x)Ga_(1−x)As (0<x<1) second cladding layer, an Al_(y)Ga_(1−y)As(0<y<1) third cladding layer, and a GaAs contact layer; and having Alcompositional ratio “z” of the etching stop layer, Al compositionalratio “x” of the first cladding layer and the second cladding layer, andAl compositional ratio “y” of the third cladding layer satisfying therelations x<z and x<y, where a difference between “x” and “z” is set to0.025 or more; and forming a stripe-patterned ridge by wet-etching thecontact layer, the third cladding layer and the second cladding layer;wherein the ridge forming step further includes: a first etching step ofwet-etching part of the contact layer, the third cladding layer, and thesecond cladding layer; and a second etching step of etching the residualportion of the second cladding layer up to the etching stop layer, byusing an etchant comprising a mixed solution of an aqueous citric acidsolution and an aqueous hydrogen peroxide solution.

In the method according to the preferred embodiment of the presentinvention, the third cladding layer and the GaAs contact layer, whichare hard to be etched by the etchant which comprises a mixed solution ofan aqueous citric acid solution and an aqueous hydrogen peroxidesolution, are wet-etched in the first etching step typically by using asulfuric-acid-containing etchant, and the residual section of the secondcladding layer is then etched by using an etchant having an etchingselectivity over the etching stop layer to thereby form the ridge.

The etching in the second etching step can be proceeded with anadvantageous in-plane uniformity even if any non-uniformity should occurin the first etching step due to a poor in-plane etching uniformity, sothat the semiconductor laser element successfully exempts from beingvaried in the characteristics.

According to the method of the preferred embodiment of the presentinvention, provision of the AlGaAs layer having thus-specified Alcompositional ratio as the etching stop layer makes it possible toensure a sufficient etching selectivity even if the etching stop layeris formed to as thin as 0.015 μm to 0.02 μm, and thereby facilitates theridge formation.

According to the method of the preferred embodiment of the presentinvention in which the etching stop layer is composed ofAl_(x)Ga_(1−z)As (0<z≦1) similarly to the cladding layer, it is no morenecessary to alter the furnace temperature for growing the etching stoplayer in the first epitaxial growth step, and this makes it possible toproceed epitaxial growth of all of the first cladding layer, etchingstop layer, second cladding layer, third cladding layer and contactlayer in a succeeding manner. In short, the method according to thepreferred embodiment of the present invention is successful in readilyforming a stacked structure of the semiconductor laser element within ashort time, and in realizing the semiconductor laser element at lowcosts.

According to another preferred embodiment of the present invention, thestacked structure etched in the first etching step is transferred fromthe first etching step through the cleaning step to the second etchingstep without exposing it to the air and water. Transfer of the waferwithout exposing it to the air and water can be accomplished typicallyby transferring the wafer to an inert gas atmosphere or to vacuumatmosphere.

This successfully prevents the wafer from being exposed to the air andwater during the ridge formation process, realizes a fabrication processcapable of suppressing natural oxidation of the wafer surface, and thusrealizes a good reproducibility in the process without causing whiteclouding of the wafer and non-uniformity in the etching.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description ofthe presently preferred exemplary embodiment of the invention taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view showing a configuration of asemiconductor laser element according to a preferred embodiment of thepresent invention;

FIGS. 2A and 2B are cross-sectional views showing layer structures inthe individual process steps in the fabrication of the semiconductorlaser element according to a preferred embodiment of the presentinvention;

FIGS. 3C and 3D are cross-sectional views showing layer structures inthe individual process steps in the fabrication of the semiconductorlaser element according to a preferred embodiment of the presentinvention, as continued from FIG. 2B;

FIGS. 4E and 4F are cross-sectional views showing layer structures inthe individual process steps in the fabrication of the semiconductorlaser element according to a preferred embodiment of the presentinvention, as continued from FIG. 3D;

FIG. 5 is a cross-sectional view showing layer structure in a processstep in the fabrication of the semiconductor laser element according toa preferred embodiment of the present invention, as continued from FIG.4F;

FIG. 6 is a graph showing etch rates of Al_(x)Ga_(1−x)As layers using anetchant comprising a 12:1 mixed solution of citric acid monohydrate (50wt % aqueous solution) and an aqueous hydrogen peroxide solution (31%);

FIG. 7 is a drawing of energy levels of a layer structure of thesemiconductor laser element according to a preferred embodiment of thepresent invention;

FIG. 8 is a drawing of energy levels of a layer structure of aconventional semiconductor laser element;

FIG. 9 is a graph showing etchrates of Al_(x)Ga_(1−x)As layers using anetchant comprising an 11:1 mixed solution of citric acid monohydrate (50wt % aqueous solution) and an aqueous hydrogen peroxide solution (31%);

FIG. 10 is a cross-sectional view showing a configuration of aconventional semiconductor laser element;

FIGS. 11A and 11B are cross-sectional views showing layer structures inthe individual process steps in the fabrication of a conventionalsemiconductor laser element;

FIGS. 12C and 12D are cross-sectional views showing layer structures inthe individual process steps in the fabrication of a conventionalsemiconductor laser element, as continued from FIG. 11B; and

FIGS. 13E and 13F are cross-sectional views showing layer structures inthe individual process steps in the fabrication of a conventionalsemiconductor laser element, as continued from FIG. 12D.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTIONN

The following paragraphs will specifically describe preferredembodiments of the present invention in detail with reference to theattached drawings. It is to be understood that the film forming method,composition and thickness of the compound semiconductor layers andprocess conditions described in the preferred embodiments of the presentinvention below are none other than exemplary ones for the convenienceof understanding of the present invention, and by no means limit thepresent invention.

Examples of Embodiments of the Semiconductor Laser Element

This example of embodiment relates to an example of semiconductor laserelement according to a preferred embodiment of the present invention.FIG. 1 is a cross-sectional view showing a configuration of asemiconductor laser element according to such embodiment.

A semiconductor laser element 100 of this embodiment comprises a stackedstructure formed on an n-GaAs substrate 101, where the stacked structurecomprises an n-GaAs buffer layer 102, an n-Al_(0.6)Ga_(0.4)As claddinglayer 103, an n-Al_(0.47)Ga_(0.53)As cladding layer 104, an active layersection 105, a p-Al_(0.47)Ga_(0.53)As first cladding layer 106, anAl_(0.55)Ga_(0.45)As etching stop layer 107, a p-Al_(0.47)Ga_(0.53)Assecond cladding layer 108, a p-Al_(0.6)Ga_(0.4)As third cladding layer109, and a p-GaAs contact layer 110 stacked in this order.

The p-Al_(0.47)Ga_(0.53)As second cladding layer 108, thep-Al_(0.6)Ga_(0.4)As third cladding layer 109, and the p-GaAs contactlayer 110 are formed as a stripe-patterned ridge so as to constitute acurrent injection region 120. Both side faces and both lateral portionsare filled with an n-GaAs current blocking layer 111 to thereby formnon-current-injection regions 121.

A p-side electrode 112 is formed on the upper surfaces of the p-GaAscontact layer 110, which forms the upper surface of the ridge, and then-GaAs current blocking layer 111 and an n-side electrode 113 is formedon the back surface of the n-GaAs substrate 101.

As an example of this preferred embodiment, the thickness of the n-GaAsbuffer layer 102 is set to 0.5 μm, the n-Al_(0.6)Ga_(0.4)As claddinglayer 103 to 1.0 μm, the n-Al_(0.47)Ga_(0.53)As cladding layer 104 to0.6 μm, the p-Al_(0.47)Ga_(0.53)As first cladding layer 106 to 0.3 μm,the Al_(0.55)Ga_(0.45)As etching stop layer 107 to 0.015 μm, thep-Al_(0.47)Ga_(0.53)As second cladding layer 108 to 0.3 μm, thep-Al_(0.6)Ga_(0.4)As third cladding layer 109 to 1.0 μm, and the p-GaAscontact layer 110 to 0.5 μm.

The active layer section 105 of this example of preferred embodiment ofthe present invention is configured so as to have an SCH (SeparatedConfinement Heterostructure) which is composed as an optical waveguidelayer including an optical guide layer, and is more specificallyconfigured so as to have an SQW (Single Quantum Well) structure whichcomprises a 0.05-μm thick Al_(0.3)Ga_(0.7)As optical guide layer and a0.01-μm thick Al_(0.1)Ga_(0.9)As active layer.

It is to be noted that configuration of the active layer section 105 isnot limited thereto, allowing any other designs or configurations.

According to this example of preferred embodiment of the presentinvention, each of the upper and lower cladding layer disposed whileplacing the active layer section 105 in between respectively comprisestwo layers of an Al_(0.6)Ga_(0.4)As cladding layer and anAl_(0.47)Ga_(0.53)As cladding layer, where the cladding layers on theactive-layer-section 105 side are composed of Al_(0.47)Ga_(0.53)As, andthe outer cladding layers further sandwiching them are composed ofAl_(0.6)Ga_(0.4)As.

In this configuration, the cladding layers disposed on theactive-layer-section 105 side have a refractive index larger than thatof the outer cladding layer, so that the light leaked from the lightemitting layer section 105 can efficiently be confined in the claddinglayers on the active-layer-section 105 side.

FIG. 7 is a drawing of energy levels of a layer structure of thesemiconductor laser element 100 of this embodiment. The semiconductorlaser element 100 has a light confinement factor of 3.2164%.

On the other hand, FIG. 8 is a drawing of energy levels of a layerstructure of a conventional semiconductor of a single-layered claddinglayer structure having neither the n-Al_(0.6)Ga_(0.4)As cladding layer103 nor the p-Al_(0.6)Ga_(0.4)As third cladding layer 109, which areconstituents of the semiconductor laser element 100 of this embodiment.

The layer structure shown in FIG. 8 include a 1-μm thickn-Al_(0.47)Ga_(0.53)As cladding layer and an p-Al_(0.47)Ga_(0.53)Assecond cladding layer in place of the n-Al_(0.6)Ga_(0.4)As claddinglayer 103 and the p-Al_(0.6)Ga_(0.4)As third cladding layer 109, andcorresponds to a structure of a conventional semiconductor laserelement.

This configuration is such as having the n-Al_(0.47)Ga_(0.53)As claddinglayer and the p-Al_(0.47)Ga_(0.53)As cladding layer while placing theactive layer in between, but having no additional Al_(0.6)Ga_(0.4)Ascladding layers, being a lower-refractive-index material, furthersandwiching these cladding layers, and has a light confinement factor of3.1645%. In other words, the conventional semiconductor laser elementcan achieve a light confinement effect only to a weaker degree ascompared with that this example of preferred embodiment of the presentinvention.

Because the Al_(0.55)Ga_(0.45)As etching stop layer 107 in the presentembodiment is configured as having the thickness reduced to as thin as0.015 μm, the refractive index of the etching stop layer 107 becomesless affective to the light generated in the active region, and thissuccessfully reduces influences on the optical characteristics of thesemiconductor laser element.

In the present embodiment, difference in the Al compositional ratiobetween the p-Al_(0.6)Ga_(0.4)As third cladding layer 109 and thep-Al_(0.47)Ga_(0.53)As second cladding layer 108 is 0.13, and differencein the Al compositional ratio between the Al_(0.55)Ga_(0.45)As etchingstop layer 107 and the p-Al_(0.47)Ga_(0.53)As second cladding layer 108is 0.08. This facilitates the formation of the stripe-patterned ridge asdescribed in the fabrication method below.

Examples of Preferred Embodiments for the Method of Fabricating aSemiconductor Laser Element

This embodiment relates to one exemplary preferred embodiment of amethod of fabricating a semiconductor laser element according to thepresent invention. FIGS. 2A and 2B, FIGS. 3C and 3D, FIGS. 4E and 4F,and FIG. 5 show cross-sectional views of layer structures in theindividual process steps in the fabrication of the semiconductor laserelement of the above-described embodiment.

First Epitaxial Growth Step

First, as shown in FIG. 2A, the n-GaAs buffer layer 102, then-Al_(0.6)Ga_(0.4)As cladding layer 103, the n-Al_(0.47)Ga_(0.53)Ascladding layer 104, the active layer section 105, thep-Al_(0.47)Ga_(0.53)As first cladding layer 106, theAl_(0.55)Ga_(0.45)As etching stop layer 107, the p-Al_(0.47)Ga_(0.53)Assecond cladding layer 108, the p-Al_(0.6)Ga_(0.4)As third cladding layer109, and the p-GaAs contact layer 110 are epitaxially grown sequentiallyin this order on the n-GaAs substrate 101 in the first epitaxial growthstep by an organometallic vapor phase growth process such as the MOVPEprocess and MOCVD process, to thereby form a stacked structure 116having a double hetero-structure.

In the epitaxial growth, Si, Se and so forth are used as the n-typedopant, and Zn, Mg, Be and so forth as the p-type dopant.

Ridge Formation Step [Pre-Process]

Next, as shown in FIG. 2B, an SiO₂ film 114 is formed on the top surfaceof the stacked structure 116, that is, the upper surface of the p-GaAscontact layer 110, by the CVD (Chemical Vapor Deposition) process or thelike, and further on the SiO₂ film 114, a stripe-patterned resist mask115 is formed by photolithography.

Next, as shown in FIG. 3C, the SiO₂ film 114 is patterned by an etchingtechnique using the resist mask 115, to thereby form a stripe-patternedSiO₂ mask 114. After the SiO₂ mask 114 is formed, the resist mask 115 isremoved.

Next, the p-GaAs contact layer 110, the p-Al_(0.6)Ga_(0.4)As thirdcladding layer 109, and the p-Al_(0.47)Ga_(0.53)As second cladding layer108 are etched by a two-step wet etching technique under masking withthe SiO₂ film 114, to thereby form a stripe-patterned ridge.

[First Wet Etching Step]

In the first wet etching step, as shown in FIG. 3D, the p-GaAs contactlayer 110 is etched typically by using a sulfuric-acid-containingetchant, and then the p-Al_(0.47)Ga_(0.53)As second cladding layer 108is etched so as to terminate the etching in halfway by controlling theetching time, so as to prevent the etching from reaching theAl_(0.55)Ga_(0.45)As etching stop layer 107.

It is to be noted that it is very difficult for thesulfuric-acid-containing etchant to ensure an etching selectivitybetween the p-Al_(0.47)Ga_(0.53)As second cladding layer 108 and theAl_(0.55)Ga_(0.45)As etching stop layer 107.

A composition of the etchant used in the first wet etching step issulfuric acid (96%): hydrogen peroxide (31%): water=1:8:40 (in ratio byvolume, compositional ratio of the etchant mixed solution will beexpressed in ratio by volume unless otherwise specifically noted), andan etching time is set to 2 minutes. By terminating the etching to aslong as this duration of time, the p-Al_(0.47)Ga_(0.53)As secondcladding layer 108 is remained in a thickness of approximately 0.2 μm onthe Al_(0.55)Ga_(0.45)As etching stop layer 107.

The sulfuric-acid-containing etchant has poor reproducibility and isdifficult to secure on-plane uniformity. More specifically, the etchantcan etch the p-Al_(0.47)Ga_(0.53)As second cladding layer 108 only witha poor in-plane uniformity, and thereby causes a large in-planevariation in the degree of the etching. If the variation remainsuncorrected, the resultant ridges will have heights differing fromelement to element within the wafer plane, and this will be causative ofvariations in the difference in the refractive indices and emissionangle characteristic. According to this example of preferred embodimentof the present invention, this is solved by the second etching stepdescribed later.

After completion of the first etching step, the etched stacked structureis dipped into a cleaning solution containing citric acid monohydrate(50 wt % aqueous solution), without subjecting it to cleaning underrunning water and drying, and also without disposing it to the air andwater, and the cleaning solution is stirred to thereby remove thesulfuric-acid-containing etchant remained on the wafer surface. Thestirring time is adjusted to 20 seconds.

The wafer is preferably conveyed while being kept away from the air andwater in order to avoid drying and the consequent natural oxidation ofthe wafer surface.

Next, in order to thoroughly remove the resultant native oxide film onthe wafer surface and the residual sulfuric-acid-containing etchant, thewafer is subsequently dipped into another fresh section of the cleaningsolution containing citric acid monohydrate (50 wt % aqueous solution),and the cleaning solution is stirred to rinse the wafer. The stirringtime is adjusted to 1 minute.

Also when the wafer is transferred to a new cleaning solution, it ispreferable to avoid exposure of the wafer surface to the air and waterin order to prevent natural oxidation of the wafer surface due todrying. Also cleaning under running water is not desirable.

[Second Wet Etching Step]

Next, the process advances to a second wet etching step. In the secondetching step, a mixed solution of citric acid and an aqueous hydrogenperoxide solution is used as an etchant, and as shown in FIG. 4E, theresidual section of the p-Al_(0.47)Ga_(0.53)As second cladding layer 108is etched to as deep as to reach the Al_(0.55)Ga_(0.45)As etching stoplayer 107.

The etchant used in the second etching step is a mixed solution having acompositional ratio of citric acid monohydrate (50 wt % aqueoussolution) and an aqueous hydrogen peroxide solution (31%) of 12:1, andthe etching time is adjusted to 2 minutes.

The etchant comprising the mixed solution of citric acid and an aqueoushydrogen peroxide solution has an etching selectivity over theAl_(0.55)Ga_(0.45)As etching stop layer 107. That is, theAl_(0.55)Ga_(0.45)As etching stop layer 107 is abruptly oxidized as soonas it is exposed, and reduces its etchrate by the etchant, so that theetching terminates on the etching stop layer 107.

Also when the wafer is transferred to a new cleaning solution, it ispreferable to avoid exposure of the wafer surface to the air and waterin order to prevent natural oxidation of the wafer surface due todrying. Also cleaning under running water is not carried out.

FIG. 6 is a graph showing etchrates of Al_(x)Ga_(1−x)As layers using anetchant comprising a 12:1 mixed solution of citric acid monohydrate (50wt % aqueous solution) and an aqueous hydrogen peroxide solution (31%).

The graph has the abscissa representing the etching time [min] and theordinate representing the amount of etching [μm], where lines from thebottom to the top express etchrates of the AlGaAs layers having an Alcompositional ratio “x” of 0.55, 0.525, 0.5, 0.475, 0.45 and 0 (GaAs),respectively.

As it is seen from the graph of FIG. 6, the etchrate increases as the Alcompositional ratio decreases, and it was found that a large selectivitycan be secured between Al_(0.5) Ga_(0.5)As and Al_(0.525)Ga_(0.475)As.This means that an Al compositional ratio of up to 0.5 allows etching toproceed, but an Al compositional ratio of 0.525 prevents the etchingfrom proceeding because Al binds with oxygen in the hydrogen peroxide toform aluminum oxide.

In other words, Al_(0.5)Ga_(0.5)As allows the etching effect topredominantly be expressed, but Al_(0.525)Ga_(0.475)As, having an Alcompositional ratio of only larger by 0.025, completely inhibits theetching due to a predominant effect of oxidation based on binding of Alwith oxygen in hydrogen peroxide.

The use of the above-described etchant is therefore successful inselectively etching the p-Al_(0.47)Ga_(0.53)As second cladding layer108, and in terminating the etching on the Al_(0.55)Ga_(0.45)As etchingstop layer 107.

The etching time necessary for etching the p-Al_(0.47)Ga_(0.53)As secondcladding layer 108 having a thickness of approximately 0.2 μm on theAl_(0.55)Ga_(0.45)As etching stop layer 107 can be estimated as 1 minuteor around as being judged from the graph shown in FIG. 6, but the actualetching time in the second etching step is adjusted to 2 minutes.

This is because the first etching step using thesulfuric-acid-containing etchant resulted in in-plane variation of theetching as described in the above, and it is necessary to resolve theetching variation in the second etching step by setting the etching timeslightly longer than the estimation.

As is known from FIG. 6, GaAs shows a largest etchrate. Therefore asshown in FIG. 4E, the p-contact layer 110 causes an undercut of as deepas approximately 0.2 μm under the SiO₂ mask 114.

As is also known from FIG. 6, the p-Al_(0.6)Ga_(0.4)As third claddinglayer 109 cannot be etched by the etchant used in the second etchingstep of this reason, the first etching step adopts thesulfuric-acid-containing etchant so as to allow the etching to proceedto as far as to reach the p-Al_(0.47)Ga_(0.53)As second cladding layer108.

In the ridge formation by the etching in the present embodiment, it isessential to keep the wafer surface away from the air and water. This isbecause AlGaAs can readily oxidize when exposed to an atmosphere such aswater or the air, and this outermost oxide film immediately inhibits theetching, to thereby cause clouding or non-uniformity in the etching.

Second Epitaxial Growth Step

Next, as shown in FIG. 4F, in the second epitaxial growth step, then-GaAs current blocking layer 111 is selectively grown on both lateralfaces and both lateral sections of the ridge under masking with the SiO₂mask 114.

The n-GaAs current blocking layer 111 is grown on the lateral faces andboth lateral sections of the ridge, but is not grown on the SiO₂ mask114.

Electrode Formation Step

Next, as shown in FIG. 5, the SiO₂ mask 114 is removed, the p-sideelectrode 112 is formed on the p-GaAs contact layer 110 and the n-GaAscurrent blocking layer 111, and the n-side electrode 113 is formed onthe back surface of the n-GaAs substrate 101.

A semiconductor wafer for fabricating the laser elements, of which layerstructure previously shown in FIG. 1, is thus obtained.

The semiconductor wafer for fabricating the laser elements is then cleftalong the direction normal to the stripe-patterned ridge to therebyfabricate the semiconductor laser element having a pair of reflectivesurfaces for composing an oscillator.

Although a difference in the Al compositional ratio between thep-Al_(0.47)Ga_(0.53)As second cladding layer 108 and theAl_(0.55)Ga_(0.45)As etching stop layer 107 is only as small as 0.08 inthe semiconductor laser element 100 of this embodiment, the use of thecitric-acid-containing etchant specified in the present invention cansuccessfully secure an advantageous level of the etching selectivitytherebetween, and facilitates the ridge formation.

On the other hand, because the Al compositional ratio of the etchingstop layer 107 is as large as 0.55, carrier recombination within theetching stop layer 107 is suppressed, and this makes it possible torealize a semiconductor laser element having a low element resistanceand a low operational voltage.

The use of AlGaAs for composing the etching stop layer also makes itpossible to grow this layer in the first epitaxial growth step withoutchanging the growth temperature. This is successful in readily formingthe stacked structure 116 within a short period of time, and infabricating the semiconductor laser element at low coasts.

The fabrication method according to a preferred embodiment of thepresent invention uses two or more types of etchant including a mixedsolution of an aqueous citric acid solution and an aqueous hydrogenperoxide solution for the ridge formation, and never disposes the waferto the air during the ridge formation, so that the wafer is successfullyprevented from being naturally oxidized and allows a highly reproducibleetching process to proceed without causing clouding of the surfacethereof and non-uniformity in the etching.

Although the difference in the Al compositional ratio between theAl_(0.55)Ga_(0.45)As etching stop layer 107 and thep-Al_(0.47)Ga_(0.53)As second cladding layer 108 was adjusted to 0.08according to this example of preferred embodiment of the presentinvention, these values are not specifically limited provided that thedifference in the Al compositional ratio is adjusted to 0.025 or larger,and the compositional ratio of citric acid monohydrate (50 wt % aqueoussolution) and an aqueous hydrogen peroxide solution (31%) is adjusted soas to ensure the etching selectivity.

For example, it is known from FIG. 9 that use of an etchant whichcomprises an 11:1 mixed solution of citric acid monohydrate (50 wt %aqueous solution) and an aqueous hydrogen peroxide solution (31%) cansuccessfully ensure a large etching selectivity betweenAl_(0.475)Ga_(0.525)As and Al_(0.5)Ga_(0.5)As.

FIG. 9 shows a graph showing etchrates of Al_(x)Ga_(1−x)As layers usingan etchant comprising an 11:1 mixed solution of citric acid monohydrate(50 wt % aqueous solution) and an aqueous hydrogen peroxide solution(31%), where the abscissa represents the etching time [min] and theordinate represents the amount of etching [μm], and lines from thebottom to the top express etchrates of the AlGaAs layers having an Alcompositional ratio “x” of 0.55, 0.525, 0.5, 0.475, 0.45 and 0 (GaAs),respectively.

Although the n-GaAs current blocking layer 111 is grown on the obliquefaces and both lateral sections of the ridge so as to bury the ridge,the ridge may be formed into any other geometries depending on thedesign of wave-guiding mechanism.

Although the present example has a structure in which theAl_(0.55)Ga_(0.45)As etching stop layer 107 remains in the lateralportion of the ridge, it is also possible to remove the layer.

Although the p-Al_(0.47)Ga_(0.53)As first cladding layer 106 andp-Al_(0.47)Ga_(0.53)As second cladding layer 108 have the same Alcompositional ratio in this example of preferred embodiment of thepresent invention, the ratio may differ from each other. It is stillalso allowable that the Al compositional ratio of thep-Al_(0.47)Ga_(0.53)As first cladding layer is adjusted so as to allowthe layer to serve also as the etching stop layer, or the layer may beidentical to the etching stop layer.

The substrate is not specifically limited to the n-GaAs substrate asdescribed in the above, and of course may be a p-GaAs substrate.

Therefore, although the preferred embodiments of the present inventionhave been described above in their preferred forms with a certain degreeof particularity, it should be understood by those of ordinary skill inthe art that that the present invention is not limited thereto and thatneedless to say, other various modifications, variations, combinationsand sub-combinations of such embodiments and equivalents thereof may bemade without departing from the scope and spirit of the presentinvention.

1. A method of fabricating an AlGaAs-based ridge-stripe semiconductorlaser element comprising the steps of: forming a stacked structure on anactive layer section, said stacked structure comprising anAl_(x)Ga_(1−x)As (0<x<1) first cladding layer, an Al_(z)Ga_(1−z)As(0<z≦1) etching stop layer, an Al_(x)Ga_(1−x)As (0<x<1) second claddinglayer, an Al_(y)Ga_(1−y)As (0<y<1) third cladding layer, and a GaAscontact layer, said etching stop layer being in contact with said firstcladding layer and said second cladding layer, and having Alcompositional ratio “z” of said etching stop layer, Al compositionalratio “x” of said first cladding layer and said second cladding layer,and Al compositional ratio “y” of said third cladding layer satisfyingthe relations x<z and x<y, where a difference between “x” and “z” is setto 0.025 or more; and forming a stripe-patterned ridge by wet-etchingsaid contact layer, said third cladding layer and said second claddinglayer, wherein, said ridge forming step further comprises: a firstetching step of wet-etching part of said contact layer, said thirdcladding layer, and said second cladding layer; and a second etchingstep of etching the residual portion of said second cladding layer up tosaid etching stop layer, by using an etchant comprising a mixed solutionof an aqueous citric acid solution and an aqueous hydrogen peroxidesolution.
 2. The method of fabricating a semiconductor laser element asclaimed in claim 1, further comprising a step of cleaning said stackedstructure etched in said first etching step using an aqueous citric acidsolution, between said first etching step and said second etching step.3. The method of fabricating a semiconductor laser element as claimed inclaim 2, wherein said stacked structure etched in said first etchingstep is transferred from said first etching step through said cleaningstep to said second etching step without further exposure to water andexposure to air so as to minimize natural oxidation of the wafer surfacedue to drying.
 4. The method of fabricating a semiconductor laserelement as claimed in claim 1, wherein a thickness of the etching stoplayer is 0.015 pm.
 5. The method of fabricating a semiconductor laserelement as claimed in claim 4, wherein the thickness of the etching stoplayer is within a range of 0.015 μm and 0.02 μm.
 6. The method offabricating a semiconductor laser element as claimed in claim 1, whereinthe difference between “y” and “x” is 0.13, and wherein the differencebetween “z” and “x” is 0.08.
 7. The method of fabricating asemiconductor laser element as claimed in claim 1, wherein the stackedstructure is expitaxially grown sequentially on the active layersection.
 8. The method of fabricating a semiconductor laser element asclaimed in claim 1, wherein the mixed solution includes 50 percentaqueous citric acid solution and 31 percent aqueous hydrogen peroxidesolution to ensure etching sensitivity.
 9. The method of fabricating asemiconductor laser element as claimed in claim 1, wherein a thicknessof the first cladding layer is 0.3 μm, a thickness of the secondcladding layer is 0.03 μm, a thickness of the third cladding layer is1.0 μm, and a thickness of the contact layer is 0.5 μm.